UNIT 1 C
3 marks Questions:
1. Explain the
working principle of
magnetic particle inspection
testing.
A. MAGNETIC PARTICLE INSPECTION
Magnetic particle method is used for locating surface and subsurface
dis- continuities in Ferro-magnetic materials. It is based on the fact that.
when the material or the part under test is magnetized, magnetic
discontinuities that lie in a direction generally transverse to the direction
of the magnetic field, will cause a leakage field to be formed at and above the
surface of the part. The presence of this leakage field, and therefore the
presence of the discontinuity, is detected by the use of finely divided Ferro-magnetic
part1- clues applied over the surface, some of the particles being gathered and
held by the leakage field. This magnetically held collection of particles forms
an outline of the discontinuity and generally indicates its location, size,
shape and extent. Magnetic particles are applied over a surface as dry particles,
or as wet particles in a liquid carrier such as water or oil.
One of the basic requirements
of magnetic particle inspection is that the part undergoing inspection must be
properly magnetized, so that leakage fields created by discontinuities will
attract the magnetic particles. Ferro- magnetic materials can be magnetized in two
different ways, either, by the use of permanent magnets, or by passing an
electric current through the material.
A horse shoe magnet can be used to magnetize the material to be
tested. The principle of working of a horse shoe magnet is explained in the Fig..When
a magnetic material is placed across the poles of a horse shoe magnet having
square ends, forming a closed or ring-like assembly,
the magnetic lines of force flow from the north pole through the
magnetic material to the south pole and are entirely enclosed Within the
ring-like assembly, because no external poles exist. As a result. any iron
fillings or magnetic particles dusted over this assembly are not attracted to
the magnet. But if one end of the magnet is not square, an air gap exists
between that end of the magnet and the magnetic material. The poles (of the
shoe-magnet) will than attract magnetic materials and the magnetic particles
will cling-to the poles and bridge the gap between them. Similarly any radial
crack in a circularly magnetized piece will create a north and a south magnetic
pole at the edges of a crack. Magnetic particles will be attracted to the poles
created by such a crack, giving an indication of the discontinuity in the
piece.
When the magnetic flux is produced in a ferromagnetic material by pas-
sing an electric current through a coil of wire wound around the material, the
method is called Iiiagra flux inspection. Production of leakage field at the
crack in a cylindrical bar by passing current through a solenoid coil is shown
in Fig(b).
2. What is crystal imperfection? Enlist
the types of imperfection
in crystal structure?
A.CRYSTAL IMPERFFCTION
Real crystals deviate from the
perfect periodicity of atoms Which is assumed in an ideal crystal. This
deviation is chiefly responsible for the changes in the mechanical and
electrical properties of the real crystals. This deviation of atoms from an
orderly array of lattice points is termed as defect or imperfection. An understanding
of these lattice defects is very important to explain the mechanical behaviour
of metals. For example the actual strength- th of polycrystalline material is
about 10 3 to 10 5 times lower than the theoretical
strength of an ideal crystal.
Crystal defects or imperfections could be of three types—
point defects
line defects
area defects.
3. What is the difference between slip
and twin?
A.
4. Explain working
principle of ultra-sonic
testing.
A. Ultrasonic inspection is a non-destructive method in which beams of
high-frequency sound waves are introduced into the material being inspected.
The sound waves travel through the material with some attendant loss of energy
and are reflected at interfaces. The reflected beam is detected and analyzed to
define the presence and location of flaws.
Ultrasonic inspection employs
ultrasonic waves. These waves are mechanical waves (in contrast to light or
x-rays) that consist of oscillations or vibrations of the atomic or molecular
particles of a substance. Ultrasonic wave behave essentially in the same way as
audible sound waves. Most ultrasonic inspection is done at frequencies between
1 to 25 megacycles, well above the audible or sonic range, which is about 20
cycles to 20 kilo- cycles per second The directivity of ultrasonic vibrations
increases with an increase in their frequency. At a frequency, of the order of
one megacycle per second, the angle of
divergence is so small that they may be called ‘ultrasonic beams".
5. Explain working
principle of Radiography testing.
A.RADIOGRAPHIC INSPECTION :
Radiography is based on the fact that when highly penetrating rays,
such as x-rays, are passed through a metal object, they are partly absorbed by
the metal. The degree of absorption of the x-rays through the metal varies,
depending upon the lack of homogeneity of material in different areas in. the
metal. Areas having defects, such as blowholes and cracks, metal would absorb
less x-rays than the remaining areas. This varying degree of absorption of
x-rays can be determined by placing a sensitive photographic film on the other
side of the specimen.
The photographic film. is darkened differently at different places
depending upon the position of the defects in the metal. Regions more
penetrable to x-rays radiation will be darker and the areas of high absorption
will be lighter. Production of such a radiograph is shown in Fig. 16.3.
The penetration power of x-rays depends upon the wavelength of the
rays. which mainly varies from 10 to 0.1 Angstrom. The lower the wave-
length. the more penetrating the rays. Generally, these x-rays can
pass through steel specimens up to 100 mm thick, copper and its alloys up to 60
mm thick and aluminium and its alloys up to 400 mm thick.
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